The present invention relates to a seismic probe, more particularly for use in untubed drilling shafts or wells. After drilling a shaft, it is of interest to model the ground around the shaft in seismic wave velocities.
To illustrate the methods presently used for this purpose, FIG. 1 shows a section through the ground passing through the vertical axis of the drilling shaft. Into the latter, whose wall is designated 2, is lowered to different successive depths a seismic probe 4. The seismic wave is produced on the surface of the ground by firing an explosive or a seismic source 3.
A seismic probe generally comprises three main parts, namely a group of transducers, generally vertical and optionally horizontal geophones; an anchoring device making it possible to apply the probe body against the shaft well; and an electronic compartment for controlling the probe and transmitting the signals intercepted by the geophones.
In certain cases, it is advantageous to detect seismic waves moving substantially vertically and thus obtain a seismic profile of the land which is as vertical as possible. For this purpose it is necessary to have an effective anchoring or fixing of the transducers at each measuring point, in order to obtain a coupling of said transducers with the ground of an optimum nature and record all the seismic waves propagating in the ground without being disturbed by parasitic resonances.
The anchoring devices are conventionally constituted with the aid of a linear spring, an electric motor or a hydraulic jack raising one or two members 5 (cf FIG. 1), in such a way that they engage against the shaft wall the complete probe body containing the transducers.
However, the probes are largely designed to operate in a small diameter shaft (approximately 10 to 30 cm) and have a probe body sometimes more than two meters long. Therefore the shaft well must be regular at least over the length of the probe, so that the latter engages well against the wall and the geophones are well coupled with the ground.
This type of seismic probe is not very suitable for drilling shafts in an "open hole", i.e. untubed shafts, which generally have a variable diameter and numerous irregularities.